1. Field of the Invention
This invention relates to helicopter rotors having an even number of blades wherein the opposed blades have a common rotor hub attachment member in the form of a flexible spar. Such a rotor is identified herein as a cross-beam rotor. The spar is preferably fabricated of uni-directional, high tensile strength fibers bonded together to be rectangular in cross-section. The spar reacts the centrifugal load between opposite blades and may extend the full span of both blades to each tip, or may terminate with an attachment connection at a convenient radial station. The full tip-to-tip spar embodiment is generally limited to helicopter tail rotor applications, since a main rotor spar of such embodiment of 90 feet or more in length would be too cumbersome to handle in the field as a single unit. The rectangular spar section is designed to provide the desired blade stiffness in the in-plane, lead-lag direction, and also to provide a greater degree of flexibility to allow blade motions due to pitch change and flapping. Pitch motions will introduce twisting into the spar member, and flapping will introduce transverse bending. This invention is directed to the providing of a flexible supporting means for the rotor blades which will not only provide a suitable driving connection between the hub and the blade, but will permit twisting and bending of the blade spar over a greater portion of its full length, so that these deformations may be accommodated inboard of the support attachment points and pass across the rotor axis.
2. Description of the Prior Art
In its earliest form, the development efforts of the assignee of this application towards elimination of conventional rotor stack bearings in favor of a flexible cross-beam rotor took the form of the embodiment depicted in its Canadian Patent to Arcidiacono No. 951,301 filed Nov. 10, 1971 and issued July 16, 1974. In Arcidiacono, the flexible straps were sandwiched between two hub plates, and pitch inputs were introduced by means of a shell which attached to the flexible strap at a point approximately 30 percent of the rotor radius. Blade pitch motions resulted in torsional strap deflections which terminate at the hub members. The slotted hub plates into which the blade spar members are recessed provides means to impart drive between the drive shaft connected hub plates and the blades. The U.S. Pat. No. 3,484,174 to McCoubrey illustrates a second embodiment of a cross-beam rotor with a gimbaled flapping hinge. The connection of the flexible member to the hinge member provides for similar torsion deflection means as in Arcidiacono in that the spar portions are rigidly clamped. Drive means is imparted from the drive shaft through a splined connection in the yoke assembly to an irregularly shaped insert and collar adapted to bear against the center portion of the spar in its spread apart relationship as it passes around the rotor axis.
The assignee's U.S. Pat. No. 3,874,820 to Fenaughty illustrates a further development in the field of this invention and is addressed principally to the angular relationship between the airfoil and supporting spar member. While the blade supporting structure is not shown, the structure utilized for flight aircraft comprised upper and lower clamp members which functioned similar to the structure shown in the Arcidiacono Patent. A still further development is presented in assignee's U.S. Pat. No. 4,008,980 Hibyan and Noehren issued Feb. 22, 1977, which shows the support structure referred to in the above reference to the Fenaughty patent. This structure was also incorporated into the assignee's YUH-60A prototype helicopters and is further described in the paper entitled: "Composite Bearingless Tail Rotor For UTTAS" dated May 1976 by R. Fenaughty and W. Noehren presented at the 32nd Annual National V/STOL Forum of the American Helicopter Society. The U.S. Pat. No. 3,874,815 to Baskin advanced the field of art by introducing flexible transverse pads in place of either the rigid clamps of Arcidiacono and others, or the metal gimbal bearings of McCoubrey. By means of the pad or flexure member located transverse to the spar member, flexibility is provided to allow bending deflections caused by blade flapping to take place inboard of the radial position of the flexure member; a feature not provided for by the fully clamped hub of Arcidiacono or McCoubrey. Similar to Arcidiacono, the spar members of Baskin are captured on their edges between bolted hub plates, the lower of which is connected to the drive shaft.